Process of making polyolefin fibers

ABSTRACT

An improved process of manufacturing fibers by the technique of forming a mixture of polymer, solvent for such polymer and, optionally, water or other flashing aids, at a temperature (flash temperature) which is high enough to bring the polymer to a plastic state and which will permit substantially complete vaporization of the solvent when the mixture is flashed, flashing the mixture into a flash zone to produce a fibrous product and subsequently refining the fibrous product characterized in that the fibrous product is first cooled prior to passing it through a primary refining zone. The primary refining is effected under conditions that impart a vigorous fibrillating action to the fibrous product and provides a pulp having a drainage factor in excess of 1.0 seconds/gram.

United States Patent 1191 Yonemorl Nov. 18, 1975 PROCESS OF MAKINGPOLYOLEFIN 3,770,856 11/1973 Ueki et a1. 264/13 FIBERS PrimaryExaminer-S. Leon Bashore [75] Inventor. Hayato Yonemorl, Iwakum, JapanAssistant Examiner peter Chin [73] Assignee: Crown ZellerbachCorporation, San Attorney, Agent, or Firm-Stanley M. Teigland; I

Francisco, Calif- Corwin R. Horton; Robert E. Howard [22] Filed: Apr.23, 1973 [21] App]. No.1 353,859 L d ABETRACJ fb b h n lmprove process 0manu acturm 1 ers t e Related Apphcatlon Data technique of forming amixture of polyn ier, solved t for Continuation-impart of Ba o- Oct suchpolymer and, optionally, water or other flashing 1972- aids, at atemperature (flash temperature) which is high enough to bring thepolymer to a plastic state and U.S-

permit substantially complete vaporization [5 II!- (:I- of the olventwhen the mixture flashed flashing the Field of Search 157 R, 157 C, 164,mixture into a flash zone to produce a fibrous product 1,21, 205, andsubsequently refining the fibrous product charac- 260/94.9 F terized inthat the fibrous product is first cooled prior to passing it through aprimary refining zone. The pri- References Cited mary refining iseffected under conditions that impart UNITED STATES PATENTS a vigorousfibrillating action to the fibrous product 2,789,903 4/1957 Lukman eta1... 162/157 R and Provides a P p having a drainage faCtOr in eXcess3,402,231 9/1968 Bynum etal 162/157 R of 3,743,272 7/1973 Nowotney etal.... 162/157 R 9 Cl 2 D 3,743,570 7/1973 Yang et al 162/157 R rawmggums PROCESS OF MAKING POLYOLEFIN FIBERS CROSS-REFERENCE TO RELATEDAPPLICATION This is a continuation-in-part of application Serial No.295,339 filed October 5, 1972.

BACKGROUND OF THE INVENTION Numerous process have been proposed forpreparing synthetic fibrous materials by flashing polymer solutions ordispersions held at high temperature and pressure into a zone of reducedpressure. In various patent literature, such as GermanOffenlegungsschrift No. 1,958,609 and Japanese patent application No.71-34921, processes are proposed in which a polymer is dissolved in asolvent therefor and heated under at least autogenous pressure and thenflashed into a zone of lower pressure to thereby vaporize the solventand form fibrous materials. In the latter mentioned patent, the fibrousmaterial thus formed is quenched with a water Spray at a temperaturebetween 60C and 80C.

Similar processes are presented in U.S. patent application Ser. No.295,339 filed Oct. 5, 1972, (assigned to the assignee of the presentapplication) and as well in German OLS No. 2,121,512 and German OLS No.2,144,409. In these processes a polymer dissolved in a solvent is mixedwith water or other liquid non-solvents for the polymer to form anemulsion of the polymer solution in a continuous water phase and thisemulsion is heated and flashed to reduced pressure zone to producefibers.

Another approach is described in U.S. patent application Ser. No.285,386, filed Aug. 30, 1973, and now abandoned (assigned to theassignee of the present application) wherein a polymer solution in whichwater is dispersed as a discontinuous phase is flashed to form fibers.In this process, the water concentration is held between 30% and 70% ofthe entire mixture and in forming the mixture is preferable to add thewater to the preformed polymer solution to insure that the water formsthe discontinuous phase.

OLS No. 2,1 17,461 describes a process in which molten polymer isemulsified with water (optionally, with a minor amount of solvent in thepolymer phase) and then heated and flashed to a reduced pressure zone toform fibers.

While many variations are evident, it is seen that the common featuresof all these fiber-making processes is the flashing into a zone ofreduced pressure of a heated mixture containing a polymer and a solventfor such polymer. Various conditions of flashing are suggested in thereferenced processes including temperature and pressure ranges for themixture to be flashed and various solvents, flashing aids, etc. Most ofthe references contemplate flashing into a zone held at atmosphericpressure. Others Suggest the possibility of flashing into a zone whichis above or below atmospheric pressure, in some case with theapplication of heat in such zone. All of the processes may lead to theproduction of fibrous material. However, each suffers fromthe'shortcoming that a specific set of conditions of flashing,particularly with regard to temperature and pressure, andinter-relationship of such conditions are not provided which will permitmanufacture, in a practical manner, of fibers having the optimumproperties desirable for their use as a synthetic pulp in themanufacture of paper by conventional techniques.

Fibrous materials produced under the general process conditionsdescribed in these references tend to be interconnected or bundledtogether to an undesirable degree and the paper produced therefrom isundesirably low in strength (e.g., tensile strength). Such fibrousmaterial is more difficult to separate, cut or refine in preparation forpapermaking and contains a high content of gels and chunks of polymerwhich cause undesirable fish eyes or transparent spots in papermanufactured therefrom.

It has recently been suggested (in a U.S. patent application Ser. No.340,140 of H. Yonemori filed by the present assignee on Mar. 12, 1973and entitled Process of Making Fibers) to overcome these problems byproviding a specific interrelated set of flashing conditions'to producea' product capable of being readily separated and refined into animproved pulp for papermaking which has a low content of gels and chunksand excellent drainage factor characteristics. Briefly, that processcomprises a flashing into a zone at subatmospheric pressure to provide afibrous product having a temperature less than C and subsequentlyrefining the pulp at this low temperature, preferably while maintainingthe pulp under such subatmospheric conditions in order to preventcondensation of solvent vapor. While this approach is completelysatisfactory from a technical point of view, it is expensive to practicecommercially because of the vacuum requirements imposed on the system.

It has been suggested (in a U.S. patent application of J. Kozlowskirecently filed by the present assignee entitled Processof Making Fibers)to obtain a fibrous pulp having high drainage characteriztics bymaintaining the flashed product at a temperature above the boiling pointof the solvent (preferably between 70C and C) subjecting the fibrousproduce to a primary refining that comprises alight defibering orshredding action, and subjecting the pulp to a vigorous fibrilatingaction in a secondary refining zone.

In Netherlands application No. 72/10567 published Feb. 3, 1973 it isdisclosed that a fibrous gel material containing large amounts ofsolvent is first subjected to disc refining in the presence of thesolvent to develop the fibers, followed by steam stripping the solventfrom the fibers in an aqueous slurry in the presence of polyvinylalcohol.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The purpose of the presentinvention is to provide an economical technique applicable to all of theflashing processes previously described which will produce a pulp usefulfor producing synthetic paper by conventional papermaking techniqueswhich has surprisingly improved properties and a low content of gels andchunks. The present process obviates the need for refining undersubatmospheric pressure.

Briefly the present process comprises taking the flashed fibrous productproduced by any of the foregoing processes, forming an aqueous slurry ofthe fibrous product at a temperature between about 10C and 60C, andpassing the fibrous product at such a temperature through a primaryrefining zone operated under conditions such that the fibrous product issubjected to a vigorous refining or fibrillating action. The resultingpulp has a drainage factor in excess of 1.0 second/ gram and as high as200 seconds/gram or higher.

DESCRIPTION OF PREFERRED EMBODIMENTS In practicing the process of thepresent invention, any polymer or copolymer may be employed which iscapable of forming fibers by conventional spinning techniques. It ispreferred to employ crystalline or partially crystalline polyolefinssuch as low pressure polyethylene, isotactic or partially isotacticpolypropylene, and ethylene-propylene copolymers. Additionally,polybutenes and polymethyl pentenes may be employed in the practice ofthis invention. Crystalline or partially crystalline polyamides andpolyesters may also be used. Noncrystalline polymers such aspolycarbonates, polysulfones, polyvinyl chloride,polymethylmethacrylate, polyacrylonitrile and polystyrene may be used.Mixtures of the foregoing with each other or other polymers may also beemployed.

The preferred polyolefins employed are those having an intrinsicviscosity above about 0.7 dl/g., which for high density polyethylenecorresponds to a viscosity average molecular weight of about 30,000 to40,000.

The polymers employed in practicing the present process may be in theform of dried powder or pellets or, prefereably, as a wet cake, slurryor solution of polyolefin in the reaction solvent as obtained afterpolymerization.

Generally, any substituted or unsubstituted aliphatic, aromatic orcyclic hydrocarbon which is a solvent for the polymer at elevatedtemperatures and pressures, which is relatively inert under theconditions of operation and which has a boiling point at atmosphericpressure that is between C and 130C, preferably between 50C and 100C,and at the flash zone pressure (substantially atmospheric) that is lessthan the softening point of the polymer may be employed in practicingthe present process. Illustrative of the solvents which may be utilizedare aromatic hydrocarbons, e.g., benzene; aliphatic hydrocarbons, e.g.,pentane, hexane, heptane and their isomers and homologues; alicyclichydrocarbons, e.g. cyclohexame; chlorinated hydrocarbons, e.g. methylenechloride, carbon tetrachloride and chloroform; higher alcohols; esters;ethers; kestones; nitriles, amides; fluorinated compounds, e.g.fluorohydrocarbons; nitro-methane; and mixturee of the above solventsand other solvents having a boiling point between about 20C and 100C atatmospheric pressure. The preferred solvent is hexane which has aboiling point of 68.7C at atmospheric pressure.

The polymer-solvent mixture may be formed by any one of several methods.One may start with a solution of polymer in solvent as it comes from asolution polymerization process, either at the same concentration,diluted or concentrated. Alternatively, one may start with a slurry ofpolymer particles in the solvent such as is produced by a slurry ofpolymerization procedure and the appropriate amount of water is added tothe slurry of vice versa. A further alternative would be to start with adry polymer powder, or granules, or a wet cake such as might be producedat some stage of solvent removal in the polymer plant and theappropriate amount of solvent is admixed therewith.

The polymer concentration relative to the solvent is not critical, thesolvent being present in an amount that is greater than 100% by weightof the polymer and sufficient to give a viscosity at the flashtemperature employed that can be easily handled. Frequently thisviscosity will be up to 3,500 centipoises. Generally the polymerconcentration will vary from about 2% to 4 about 30% by weight of thesolvent plus polymer, and preferably is in the range of about 5% toabout 15%.

In one preferred embodiment water is employed as a flashing aid. In thisembodiment the water may in a continuous phase or discontinuous phase,depending upon the amount of water added to the polymer-solvent mixtureand the manner of addition. If the water is to form the discontinuousphase it should be present in an amount less than To form a continuouswater phase the water should be present in an amount greater than 30% byvolume of the mixture and preferably between 50% and 70%. The particularmethod of mixing is not critical, but if it is desired to have the waterform a discontinuous phase it has been found to be advantageous to havethe solvent present prior to water addition since the solvent or polymersolution will form the continuous phase of the mixture to be formed.This latter approach is particularly desirable when one employs anamount of water which is neat the borderline of an inversion occurring,i.e., at the point where the amount of water is approaching that levelwhere it would form the continuous phase. Conversely, if the water isadded with or before the solvent, it will tend to form the continuousphase.

A primary function of the water is to provide energy to aid thevaporization of the solvent during flashing since it is not desirable tohave the temperature so high that there is sufficient energy imparted tothe solvent alone to effect its complete vaporization. However, theamount of water should not be so great as to require the expenditure ofunnecessary heat values in attaining the desired flashing temperature,i.e., once that amount of water required to form an aqueous solution ordispersion of the agent having a suitable viscosity is determined,additional water may be employed to a certain extent since it helps tolower the mixture viscosity and aid solvent vaporization but theadditional amount need not be great.

Another function of the water is to reduce the temperature of thefibrous mass in the zone immediately following the nozzle (flash zone).The addition of water increases the total vapor pressure of the systemat the moment of flashing, thus reducing the boiling point of theflashing mixture. This is independent of the amount of water employed,and very small quantities may thus be employed for this purpose. As apractical matter, however, water would be employed in the amount of atleast about 1% by volume of the solvent-water mixture. Lowering theboiling point of the mixture in this fashion will assist in establishingproper temperature conditions in the fibrous mass formed upon theflashing in accordance with this invention, as discussed in detail at alater point.

Another function of the water is to act as the carrier for a hydrophilicwater-dispersing agent for the fibers to be formed. It has been foundthat it is most advantageous to have the water-dispersing agent presentduring flashing and precipitation of the fibrous polymer. An equivalentamount of the same agent added at a later stage to the already formedfibers does not give the same degree of dispersibility and the presenceof the agent enhances the refin ability of the fibers. Therefore, ,thewater should be present in an amount sufficient to carry that amount ofthe hydrophilic agent employed to impart to the fibrous polymer thedesired level of water dispersibility, preferably as a solution thereof.Additional water above such minimum amount required to carry the agentmay be employed to impart a suitable S viscosity to the aqueous solutionor dispersion agent, 1.e., the aqueous solution of the water-dispersionagent should not be so viscous as to present problems of handling orincorporation into the polymer solution as a dispersed phase. Also, thewater can aid in reducing the viscosity of the mixture to a level lessthan that of the polymer solution alone, thus permitting higher polymerconcentrations.

The agents which may be added to the mixture to impart waterdispersibility to the fibrous polymer are preferably water-soluble orpartially water-soluble high molecular weight materials. However, theymay also be materials which are soluble or partically soluble in thesolvent so long as they are somewhat hydrophilic and impart waterdispersibility to the fibers. The amount of water-dispersing agentemployed may range from about 0.1% to about by weight of the polymer,

preferably from about 0.1% to about 5% by weight. The preferredwater-dispersing agent is an at least partially water-soluble polyvinylalcohol (PVA) having a degree of hydolysis greater than about 77% and,preferably, greater than about 88 mol.% having a viscosity (in a 4%aqueous solution at C) between about 2 and 50 centipoises. Desirably thePVA has a degree of polymerization in the range of 200 and 4000 andpreferably between 300 and 1500. If desired, the PVA may be chemicallymodified to enhance its adhesion to the polymer, dispersion andotherproperties. The polyvinyl alcohol is preferably added with the water atthe time the mixture is formed. Illustrative of other water-dispersingagents that may be employed are cationic guar, cationic starch, potatostarch, methyl cellulose and Lytron 820 (a styrene-maleic acidcopolymer).

The use of polyvinyl alcohol is important in developing the fiberproperties during refining of the flashed product in accordance withthis invention as described at a later point. For this purpose, thepolyvinyl alcohol may be added subsequent to flashing, such as with thedilution water for refining. It is particularly advantageous to add atleast 0.75% and preferably between 1%% and 5% by weight of the flashedpolymer of polyvinyl alcohol, either before and/or after flashing (butprior to refining). If polyvinyl alcohol is not present during refining,the refining can be carried out only with difficulty, if at all, and theproperties of the resulting pulp and sheets made therefrom are verypoor.

The ingredients of the mixtures can be placed in any suitable vesselwhich is capable of being heated to an elevated temperature andpressure. Generally, an autoclave is employed. However, when water isadded as a flashing aid it is important that the vessel employed beequipped with a mixing or stirring device capable of keeping the mixturein a constant state of agitation since a stable emulsion is not formedand upon standing the mixture will quickly separate into two distinctand separate phases.

The ingredients are then heated to a suitable temperature and preferablyagitated if water is present to form a uniform mixture wherein water ispresent as a discontinuous or dispersed phase within a continuous phaseof polymer solution or as a continuous phase with polymer solutiondispersed uniformly therein, depending upon the water concentration andmode of addition as previously discussed. The temperature employed ispreferably above the melt dissolution temperature of the polymer in thesolvent employed. The melt dissolution temperature of any particularsolvent is determined by placing low concentrations of the polymer theoperating conditions so that the polymer is dissolved in the solvent orat least is in a swollen state with sufficient fluidity to be dischargedfrom the nozzle, i.e. in a plastic state.

Flashing is preferably carried out substantially adiabatically,utilizing the heat (enthalpy) in the heated mixture to provide the heatof vaporization for vaporizing substantially all of the solvent when themixture is discharged to the flash zone held at a suitable lowerpressure. Accordingly, for adiabatic flashing the temperature of mixtureprior to flashing should be high enough to provide sufficient heatcontent or enthalpy for vaporization adiabatically of substantially allsolvent upon flashing to the flash zone. However, the maximumtemperature employed should be less than the critical temperature of thesolvent and/or the decomposition temperature of the polymer.

It is also possible to carry out the flashing to some extent in anon-adiabatic fashion, for example, by the addition of heat to thematerial as it is flashed from the nozzle. For instance, low pressuresteam (e.g., below 20 psig) or water at about 100C may be added to thefibrous noodle in the flashing zone as by injection thereof in a conduitimmediately following the flash nozzle into which the flashed noodle isalso injected. In this case, the flashing'temperature should be chosenso that the heat content in the mixture to be flashed plus the heatadded to the flashed material is sufficient to vaporize substantiallyall of the solvent in the flash zone.

The pressure employed in the vessel containing the heated mixture ispreferably substantially autogenous although pressures higher thanautogenous may be employed. It may be desirable, particularly in batchoperations, to employ an inert gas such as nitrogen during the flashingoperation to maintain substantially autogeneous pressure in the vesseland thus maintain the velocity of the mixture through the nozzle at afairly constant level.

Flashing is preferably effected through a nozzle which has a substantiallongitudinal dimension in order to efficiently impart shear to themixture (particularly the polymer component thereof) immediately priorto flashing. Such shearing action aids fiber formation and enhancesfiber properties for papermaking purposes. The nozzle may be circular ornoncircular in cross-section and may be an annulus.

The flash zone is maintained at a pressure which, in conjunction withthe other conditions of flashing are selected so that the temperature ofthe flashed product is almost immediately lowered in the flash zone, byevaporation of substantially all of the solvent (and a portion of theflashing aids, if employed), to a temperature below the softeningtemperature of the polymer, preferably below about C.

In a typical flashing procedure in accordance with this inventionvaporization may be substantially complete 10 to 100 cm downstream ofthe nozzle. However, this can vary widely depending on the flowvelocity, flash zone pressure, flash temperature, solvent, etc.

importantly, in the mixture to be flashed, all of the components of themixture and the concentration of each in the mixtures are chosen withrespect to the heat capacity of each, with respect to the heatvaporization of each component which will be volatized in flashing, andwith respect to the flash temperature chosen so as to produce in theflashed product a temperature above the boiling point of the solvent butpreferably less than about 100C upon flashing of the mixture into theflash zone. Expressed in another way, the heat content of the mixture tobe flashed and the heat to be removed through vaporization of thevaporizable components (solvent and any flashing aids vaporized, ifemployed) should be adjusted so that the residual heat in the flashedproduct, after removal of the heat of vaporization of the vaporizedcomponents, will impart a temperature in the flashed product which isbetween the boiling point of the solvent and about 100C. Selection ofthe appropriate flashing temperature and of the components of themixture and their concentrations for this purpose will depend upon thepressure selected for the flash zone and the amount of heat added to theflash material during flashing if the flashing is performednon-adiabatically. Preferably the flash zone is at substantiallyatmospheric pressure. However, it can be at subatmospheric pressure,(i.e., below about 600 mm Hg) as disclosed in copending application Ser.No. 340140. If the flash temperature is too high or if the components ofthe flash mixture and their concentrations with respect to their heatcapacity and heat of vaporization (for the vaporizable components) areimproperly chosen, the temperature of the flashed product, uponsubstantially complete evaporation of the solvent, will remain above lC.If the flash temperature is too low, or, again, the components areimproperly chosen with respect to heat capacity and heat ofvaporization, there will be incomplete evaporation of the solvent. Forthe purpose of this invention appropriate selection of these variableparameter, namely, the flash temperature, components and concentrationsthereof in the flash mixture, may be determined for a given pressurecondition in the flash zone by making a heat balance for the flashingoperation which will produce the desired noodle temperature below 100C.Advantageously, these variable parameters may be selected so as tosatisfy the following equation:

Q Enthalpy of polymer(s) in flash mixture Q Enthalpy of solvent(s) inflash mixture Q Enthalpy of flashing aid(s) in flash mixture Q Heatadded to flashing mixture (if nonadiabatic) V Enthalpy of vaporizationof solvent vaporized V Enthalpy of vaporization of flashing aidsvaporized (if any).

W Weight of polymer in the flashed product C',, Heat capacity of polymerin flashed product W' Weight respectively, of the flashing aids,adjuvants, and/or non-volatile components other than polymer in theflashed product (if any).

8 C, Heat capacity of the respective flashing aids, adjuvants, and/ornon-volatile components other than polymer in the flashed product (ifany).

T Boiling temperature of the solvent at the flash zone pressure. andwhere such enthalpy values are basaed on the same temperature datumplane and the heat capacities are those applicable between such datumplane and the temperature of the flashed product.

In using this formula, the heat capacity values and weights of the flashcomponents may be substituted into this formula, for example, as followsfor the specific case where a single polymer, solvent and flashing aidare present in the flash mixture:

Where the additional terms are:

W, Weight of the solvent in the flash mixture C, Heat capacity of thesolvent in the flash mixture W Weight of the flashing aid in the flashmixture C Heat capacity of the flashing aid in the flash mixture C Heatcapacity of the polymer in the flash mixture H;= Heat of vaporization ofsolvent under flash conditions.

H, Heat of vaporization of flashing aids under flash conditions.

T Flash temperature, C and where the indicated heat capacities are thoseapplicable between the flash temperature and the previously mentionedtemperature datum plane.

In practice, the heat capacity and enthalpy of vaporization values forthe desired components can be substituted into this equation. Then, aflash temperature and the concentrations of the flash mixture componentsmay be selected relative to each other to satisfy the equation for adesired flashed product temperature. For ease of calculation it can beuseful to program these variables for computer analysis to select thedesired components and flash temperature.

It is not necessary to actually measure the noodle temperature (which isa cumbersome procedure under the flash zone conditions). All that isnecessary for control purposes is to maintain the indicated parametersfor the flash procedure at values that satisfy this equation for thenoodle temperature to be above the boiling point of the solvent butbelow C. Of course, the various parameters should also satisfy the otherconditions for proper flashing as previously discussed, e.g., the flashtemperature should be above the melt dissolution temperature,substantially all of the solvent should be vaporized, etc.

During flashing, the polymer is precipitated as a fibrous product ornoodle, which is a loose aggregation of fibers which is sometimescontinuous. The fibrous product is collected in a suitable receivingvessel, preferably one which permits the vaporized solvent to beseparated therefrom.

The fibrous product in the receiving vessel may be either substantiallydry as flashed from a polyolefin solution or as an aqueous slurry ifflashed from a dispersion or emulsion employing water as a flashing aid.However, in both cases a certain amount of unflashed residual solventwill remain with the fibrous product 9 which can be subsequentlyremoved.

In the copending application Ser. No. 340140 of H. Yonemori referred toabove, it was discovered that improved products were obtained byrefining the fibrous product at relatively low temperatures, and topromote continued vaporization of the solvent the vapor separation zoneand the refining zone hadto be maintained under subatmospheric pressure.

The present invention resides in the discovery that similarly improvedproducts can be obtained by carrying out the refining of the cooledfibrous product while it is maintained at substantially atmosphericpressure, i.e., a pressure between about 600 and 800 mm Hg.

The fibrous product is cooled to a temperature between about 10C and 60Cprior to subjecting it to primary refining it at substantiallyatmospheric pressure.

if the temperature employed is higher than about 60C,

the drainage factor of the pulp is detrimentally effected. Temperaturesbelow about 10C do not give any substantial improvement and becomeuneconomi- .cal to use. Preferably the temperature is between 10 and40C.

The manner in which the fibrous product is cooled is not critical. Thefibrous product can be produced at the proper temperature by employing aflashing zone at subatmospheric pressure (as described in copendingapplication Ser. No. 340140) or by employing a low boiling solvent, i.e.one having a boiling point less than about 60C, or by a combination of asubatmospheric pressure flash zone and low boiling solvent.

Alternativey, the fibrous product can be produced at an elevatedtemperature up to about 100C, and cooled to the appropriate temperatureby use of cold dilution water, external cooling means, or a combinationof these or other conventional cooling methods.

Prior to passing the fibrous product through such primary refining zone,dilution water is added in the receiving vessel'to provide an aqueousslurry of the fibrous product having an appropriate consistency.Typically, the consistency of the squeous slurry is between about 1 to10% by weight of the fibrous material although high consistenciesbetween about 10% and 60% by weight may be employed by using screwfeeding or similar feeding means. The dilution water temperature shoulddesirably be such as to provide an aqueous slurryhaving a temperaturebetween about C and 40C, although external cooling can be employed tolower the slurry temperature to within this range.

The aqueous slurry of fibrous product at the appropriate consistency andtemperature is then passed through the primary refining zone to effectthe desired vigorous fibrillating action on the material. In itspreferred form, the primary refining zone is a disc refiner of the typeconventionally employed in the papermaking art and may employ either asingle or double rotating disc. The refiner is operated to impart avigorous fibrillating action to the fibrous product passingtherethrough. This is accomplished by spacing the discs relatively closetogether, that is, by a distance between about 0 and 2500 microns,preferably between about 0 and 200 microns. The moving disc or discs arerotated at speeds to provide relative peripheral velocities betweenabout 4000 and 8000 feet per minute. The size of the discs and the platedesign may be any of those normally employed in the papermaking art. Theprimary refining may be carried out by passing the fibrous material oneor more times through the refining zone. The vigorous fibrillatingaction may be characterized by the amount of work done on the fibers inthe primary refining zone. The work imparted to the fibrous productpassing through the primary refining zone is desirably greater thanabout 0.2 Kilowatt-hour/Kilogram.

The refining action just described may also be characterized by thefiber fractionation as carried out in accordance with TAPPI StandardTest T.-233- SU64, wherein 0.5% by weight of the fiber in an aqueousslurry is fractionated for 20 minutes in a Bauer-McNett classifierequipped with 20, 35, 65, 150 and 270 Tyler mesh screens. Afterrefining, the amount of fiber left on the 20 plus 35 mesh Tyler screensshould be less than by weight and preferably less than about 60% byweight of the sample. I

The aqueous slurry of fibrous product reslting from the primary refiningoperation drops from the bottom of the disc refiner into a receivingvessel which may be purged with an inert gas such as nitrogen to removeany solvent vaporized during the primary refining step.

The resulting pulp may be subjected to further or secondary refining ifdesired.

By employing the refining sequence just described, i.e., primaryrefining of the fibrous material as a reduced temperature a pulp isproduced having superior qualities. In this respect, one indication ofthe improvement of the fibers made by the process of the presentinvention is the drainage factor of such fibers as compared to fibers ofsimilar classified fiber length which are made using parameters outsidethe present invention. The drainage factor is a measure of the drainagecharacteristics of a fiber when a slurry thereof is placed on aforaminous surfce. For synthetic fibers of similar fiber length made bythe flashing process, important strength properties thereof correlategenerally with their drainage factor. For a fiber pulp having the sameclassified fiber length, various strength properties of paper madetherefrom increase with the drainage factor of such fibers.

Another indication of improvement of the fibers made in accordance withthis invention is their slenderness relative to fibers prepared bytypical conventional technique. It is desirable to produce fibers whichare relatively thin (or of low coarseness) as these fibers will imparthigher capcity, density and better formation to the paper preparedtherefrom.

Generally, the set of operating parameters of the present invention willresult in improved fiber properaties such as thinness and drainagefactor compared with fibers prepared using typical conventionalparameters. Because other process variables in addition to the specificparameters of this invention also influence fiber properties (e.g.,flash nozzle size and configuration, polymer type and molecular weight,solvent, flashing aids, dispersants, etc.), such resulting fibercomparasons are appropriately made with the other process variables heldconstant.

For the same reason, i.e., the influence of other process conditionsbesides the parameters specific to this invention on the resulting fiberproperties, no absolute fiber property values can be assigned to thefibers which may be prepared by the process of this invention. However,with appropriate selection of all parameters, it has been found thatpulps can be produced in accordance with this invention which have adrainage factor in excess of 1 often in excess of 10 and as high as to200 seconds per gram or higher and which have an average coarsenessbelow about 15 decigrex (m/ 100 1 1 m). For papermaking use, a drainagefactor between 2 and 10 may be the preferred range as an appropriatebalance between increased strength and ease of water removal from thefibers. Higher drainage factors can be obtained and may be useful wherethe enhanced strength is more important than rapid water removal.

In the accompanying drawing,

FIG. 1 represents a schematic representation of apparaus suitable foruse in carrying out the process of this invention, and

FIG. 2 is a detailed representation of the flash nozzle schematically.shown in FIG. 1.

In FIG. 1, l is a steam jacketed vessel, provided with an agitator 1a,which may be charged with solvent, polymer (or polymer solution) and, ifdesired, flashing aids such as water. Conduit 2 is provided at thebottom of the vessel 1 in communication with flash nozzle 3 throughshut-off valve 4 (preferably a ball valve). As seen in FIG. 2, flashnozzle 3 has a substantially smaller diameter than conduit 2. After themixture is heated to the desired temperature and agitated, if necessary,to dissolve the polymer and/or disperse the flashing aids, valve 4 isopened and the mixture thus formed is forwarded from vessel 1 throughflashing nozzle 3 under autogenous pressure of the heated mixture. Asthe mixture discharged from vessel 1, nitrogen or other inert gas may beintroduced into the head space thereof through line 5 in order tomaintain the pressure in the vessel at autogenousor higher pressure. Themixture is flashed through nozzle 3 into the flash zone which iscomprised of a vapor separation vessel such as cyclone 6, and connectingconduit 7. Conduit 7 is a pipe having an internal cross-sectional arealarger than that of flash nozzle 3 and of sufficient internal diameterto permit rapid, unrestricted passage of the flashed noodle to cyclone 6and preferably has an internal cross-sectional area many times largerthan that of the nozzle 3. Op-

tionally, low pressure steam may be introduced into conduit 7 throughline 8 located shortly beyond flash nozzle 3 to aid in the vaporizationof solvent from the precipitated noodle.

Vaporized solvent and water, leaving cyclone 6 pass through line 21 to asolvent recovery unit (not shown).

The flashed fibrous noodle entering cyclone 6 through conduit 7,together with the unvaporized portion of water or other flashing aidpresent, passes downward through conduit 9 into primary disc refiner 10.Conduit 11 is provided for introduction of dilution water at anappropriate temperature to form a mixture or slurry with the fibrousmaterial having an appropriate temperature and consistency for discrefining.

The material leaving the primary refiner passes through conduit 12 tofirst receiving rank 13. Nitrogen is introduced into tank 13 throughline 14 which sweeps out solvent vapors which are removed via suitablemeans not shown.

The slurry may be withdrawn from receiving tank 13 through line 15utilizing pump 16, for secondary refining in disc refiner 17. Thefibrous slurry leaving secondary refiner 17 passes through conduit 18 tosecond receiving tank 19 where it is collected prior to furtherprocessing for shipment or for papermaking use. If desired, the fibrousslurry leaving refiner 17 may be recycled through line 20 for one ormore additional passes through refiner 17.

The fibers after refining may be diluted to a suitable consistency andmade into synthetic paper webs either alone or blended with normalcellulose papermaking 12 fibers. Alternatively, the fibers can bedewatered, pressed into bales, stored and shipped to the ultimate user.

The illustrated apparatus may be operated on a batch basis as describedor on a continuous basis by continuously feeding vessel 1 with polymersolution and any flashing aids desired at flow rates to maintain theappropriate mixture in the vessel for flashing, while heating the vesselto maintain the mixture at the appropriate fiash temperature. In orderto insure uniform dispersion of flashing aids, it may be desirable toplace an in-line mixing device in line 2 between vessel 1 and flashnozzle 3.

Optionally, instead of using a stirred heated vessel such as vessel 1,it is also possible to prepare the mixture on a continuous basis byblending a polymer solution with any flashing aids desired (as by addingsuperheated water) continuously in an in-line mixing device 20 Justprior to flashing through a nozzle. It is also possible to utilize anin-line mixer.

The following examples will illustrate the invention.

EXAMPLE 1 The apparatus employed in this example is that illustrated inthe drawing andv previously described. The dissolution vessel 1 was a 2gallon stirred autoclave equipped with five 4 inch impellers on a singleshaft rotated at 1000 r.p.m.

The vessel was charged with 0.32 Kg of polyethylene (Mitsui 22001)having an intrinsic viscosity 1;) of 1.4 dl/gram. 4 liters of n-hexane,4 liters of water and 2% by weight of 88% hydrolyzed polyvinyl alcohol(Gelvatol 20/30 made by Monsanto) based on the polyethylene. The vesselwas sealed and the contents raised to a temperature of C 25C. The vesselwas pressurized with nitrogen to a pressure of 160 psig. The heatedcontents were then flashed through a nozzle 3 having a diameter of 2.5mm and a length of 28.5 mm into a receiving vessel 6. Dilution water ata temperature of 93C was added to the fibrous material to provide aslurry having a cosistency of about 1% by weight. The aqueous fibrousslurry at a temperature of about 40C was then passed through a primaryrefining zone consisting of a Sprout Waldron single moving disc refinerhaving 12 inch plates (Pattern C29 -79B) at a plate clearance of 0.005inch. The peripheral velocity of the refiner disc was 5495 feet perminute (1750 r.p.m.). The resulting pulp had a drainage factor of 12.6seconds/gram, a surface area (gas adsorption) of 12.7 m lgram, and afiber fraction on the 20 plus 35 mesh screen of 16%. Handsheets weremade from the pulp in accordance with TAPPI Standard Test T- 205M-58,with modified wet processing (400 psig) and a heat bonding treatment(121C at minimum pressure). The handsheets were tested and had thefollowing properties:

Basis weight, g/m 60.5 Tensile, Kg/IS mm 1.19 Breaking length, in 1309Tensile Energy Absorption, Kgcm/cm 0037 Scott Internal Bond,

Kg-cm/cm X 10 Opacity. 7c TAPPI corrected) 96.8

Handsheets were also prepared by blending the above-describedpolyethylene fibers 50/50 with bleached alder kraft pulp having aCanadian Standard I 13 Freeness of 150 c.c. The properties of theresulting handsheets were as follows:

, Basis weight.g/m

Tensile, Kg/lS mm 2.40 Breaking length, m 2677 Tensile Energy AbsorptionKg cm/cm 0.032 Scott lnternal Bond C, Kg-cm/cm X 319 Opacity. (TAPPlcorrected) 94.6

In theforegoing example and subsequent examples, the sheet density,basis weight and caliper were determined by TAPPl Standard T 220, tearstrength by TAPPl Standard T-414, opacity by TAPPl Standard T-425 rn-60,coarsenessby TAPPI Standard T-234 SU 67, and tensile strength, stretch,tensile energy adsorption and breaking length were determined by TAPPIStandard T-494.

The Scott Internal Bond was determined by employing the Scott instrumentunder the standard procedure. The drainage factor in this and followingexamples was determined substantially in accordance with' TAPPI TestT221 OS-63 with a slight modification in the method of calculation.Briefly, approximately ten grams of a fiber sample is weighed anddispersed in way 14 factor indicative of a fiber which will form a muchweaker sheet.

EXAMPLE 2 Example 1 was repeated except that the polyvinyl a1- cohol waspresent in the amount of 5 by weight of thepolyethylene. It took 76seconds to pass all the material, through the nozzle. After cooling toabout C and one pass primary refining at a plate clearance of 0.005 inchthe resulting pulp had a drainage factor of 108 seconds/gram, a surfacearea of 14.4 m /gramand a fiber fraction on the 20 p1us'35 mech screensof 55% by weight. This example illustrates that increasing the amount ofpolyvinyl alcohol during refining further enhances the drainage factorof the resulting pulp. Handsheets (both 100% and a 50/50 blend) wereprepared as in example 1. The properties were as follows:

Property 100% 50/50 Basis weight g/m 61.5 60.2

Tensile, Kg/l5 mm 1.83 3.0

Breaking length, m 1991 3321 Tensile Energy Absorption,

Kg-cm/cm 0.359 0.061

Scott lnternal Bond Opacity (TAPPl corrected) 96.1 93.1

EXAMPLE 3 Example 1 was repeated except that the temperature of thecontents of the dissolution vessel was 149C, and

low pressure steam (10 psig) was introduced into line' 7 via line 8which entered line 7 3 inches downstream of nozzle 3. Two differentkinds of polyvinyl alcohol were employed. In the first run, thepolyvinyl alcohol emwhere v DF drainage factor, seconds/gram D drainagetime with pulp in mold, seconds (1 drainage time without pulp in mold,seconds V viscosity of water at temperature T W weight of fibersemployed in test, grams.

The quantity (1/V 1) is tabulated in the aforementioned TAPPI test T221OS-63. This quantity is multiplied by 0.3 which has been empiricallydetermined for the present fibers.

COMPARATIVE EXAMPLE 1 In order to show the improvement in fiberproperties obtained by subjecting the fibrous noodle to a rigorousfibrillating action at a reduced temperature, example 1 was repeatedexcept that the primary refining was accomplished by passing the fibrousnoodle at a tempera- 'ture of 93C through the refiner with the platesset at 0.005 inch apart. The resulting pulp had a drainage factor of0.17 seconds/ gram. The resulting pulp was then cooled to about 40C asin example 1 and passed once through a secondary refiner having a plateclearance of 0.005 inch. The resulting pulp had a drainage factor ofonly 0.57 seconds/gram compared to 12.6 seconds/- gram for the pulp ofexample 1. The lower drainage ployed was Hoechst Movial 30-88 which is88% hydrolyzed and has a viscosity (4% by weight aqueous solution at20C) of 3--5 centipoises and in the second run the polyvinyl alcohol wasHoechst 30-98 which is 98% hydolysed and has a viscosity of 3-5centipoises. Pri' mary refining was effected at a temperature of 40C anda plate clearance of 0.005 inch.

The resulting pulps had the following properties:

Run PVA Drainage Factor Classified No. Hydrolyzed sec/g. Fiber length,mm

Handsheets made from the pulps, both 100% and 50/50 blends with bleachedalder kraft, as in example 1. The handhseet properties were:

Property 100% Handsheets 50/50 Handsheets PVA Hydrolysis PVA Hydrolysis88% 98% 88% 98% Basis weight g/m 63.3 62.5 58.5 57.4 Caliper, mm 0.150.16 0.11 0.11 Density g/cc 0.41 0.40 0.53 0.52 Tear. g/sheet 35.2 25.635.2 32.2 Tensile. Kg/lS mm 1.2 0.97 2.6 2.3 Breaking length, m 1.261.04 2.98 2.68

-continued Property 100% Handsheets 50/50 Handsheets PVA Hydrolysis PVAHydrolysis Stretch 7.1 5.6 3.7 3.2 Tensile Energy Absorption, Kg-cm/cm0.05 0.03 0.05 0.03 Scott internal Bond cm/cm (10"") 175 200 184 183Opacity. (TAPPl corrected) 97.6 96.6 94.7 92.6

This example illustrates that improved strength properties may beobtained with the present process by employing polyvinyl alcohols ofvarious degrees hydrolysis, hydrlysis, and that the less hydrolyzedpolyvinyl a1- cohols are preferable.

EXAMPLE 4 Example 3 was repeated using polyvinyl alcohols having highermolecular weights. The polyvinyl alcohol employed in run 1 was HoechstMovia 1 75-88 (viscosity of 25 centipoises) in 4% aqueous solution at20C, and is 88% hydrolyzed, and the polyvinyl alcohol employed in run 2was Hoechst Movial 75-98 (viscosity of -30 centipoises and is 98%hydrolyzed). The primary refining was effected at a temperature of 40Cand at a plate clearance of 0.005 inch. The resulting pulps had thefollowing properties:

Run PVA Drainage Factor Classified Fiber No. l-lydrolyzed sec/g. length.mm

Handsheets made from the pulps, both 100% and 50/50 blends with bleachedalder kraft, as in example 1 The handsheet properties were:

100% Handsheets This example illustrates that improved strengthproperties may be obtained with the present process with polyvinylalcohols of varying molecular weights.

EXAMPLE 5 The apparatus employed in this example is that illustrated inthe drawing and previously described except that ball valve 4 and nozzle3 were replaced with a valtek angle central valve with a 9.5 mm diameterorifice. The dissolution vessel 1 was an 800 gallon stirred autoclave.

The stirred vessel was charged with 300 gallons of hexane, 91 Kg ofpolyethylene (Mitsui 2200P) having an intrinsic viscosity (1)) of 1.4dl/gram, and 60 gallons of water and 3% by weight of 88% hydrolyzedpolyvinyl alcohol (Gelvatol 20/30 made by Monsanto) based on thepolyethylene (2730 grams). The vessel was sealed and the contents raisedto a temperature of 140C. The heated contents were then flashed throughthe control valve into receiving vessel 6 at a rate of about 200-250grams polyethylene per minute. Low'pressure 10 psig) steam wasintroduced into line 7 via line 8 located about 6 inches downstream ofthe control valve. Dilution water at a temperature of C was added to thefibrous material at a rate of about 5 /2 gallons/minute. Part of theaqueous fibrous slurry at a consistency of 1 and at a temperature of 85Cwas then passed through a primary refining zone consisting of at Jonesdouble disc refiner having 12 inch plates (Pattern l,1,1, /2 10) at aplate clearance of 0.002 inch (0.05 mm). The peripheral velocity of themoving refiner disc was 6688 feet per minute (2130 r.p.m.).

The slurry was passed through the refiner at a flow rate of about 15liters per minute. A second portion of the fibrous material wascollected and cooled to a temperature of about 20C prior to refining,and was then subjected to primary refining under the same conditions asthe first sample. Both samples were subjected to five passes ofsecondary refining at 20C, the other refining conditions being the sameas for the primary refining. Handsheets (both and 50/50 blends) weremade and tested as in example 1. The fiber and handsheet properties wereas follows:

Property Run 1 Run 2 85C Primary 20C Primary Refining. Refining Drainagefactor sec/g. 18.4 114.1 Fiber Fractionation on 20 mesh screen 2.3 6.735 mesh screen 38.2 25.4 65 mesh screen 23.5 28.2 mesh screen 17.7 20.1270 mesh screen 5.2 10.7 Trhough 13.1 8.9

Classified fiber Length 1.01 0.97

100% Handsheet Density. g/cc 0.44 0.44 Corr. Opacity 96.7 95.8Scattering Coefficient cmlg 90.8 91.7 Brightness 90.8 91.7 Breakinglength 1154 1855 TEA, kg-cm/cm 0.052 0.163 Internal Bond, Scott units 64167 Tear, g/sheet 19 32 Basis weight, g/m 59.9 62.2

50/50 Handsheet Density g/cc 0.58 0.57 Corr. Opacity 91.2 91.5

Scattering Coefficient cm lg 744 763 Brightness, 85.1 86.0 Breakinglength. m 3332 3545 TEA, kgcmlcm 0.051 0.066 lnternal Bond. Scott Units119 162 Tear g/sheet 30 33 Basis weight g/m 61.2 63.1

This example clearly shows the marked improvement of properties, such asdrainage factor, obtained by emy g he cold primaryrefining concept ofthe pres- 'ent invention. t

I claim:

' 1. In a method of producing a pulp of polyolefin fibers whereinanaqueous dispersion of a polyolefin in a solvent at an elevatedtemperature and pressure is flashed through a nozzle into a zone ofreduced pressure to form a fibrous product, the improvement comprisingdissolving in the aqueous phase of the dispersion from 0.75 to 10% byweight polyvinyl alcohol based on the weight of the polyolefin flashingsaid dispersion to form the fibrous product and passing the fibrousproduct at a temperature between about 10C finer with the spacingbetween the discs set at between 7 about 0' and 2500 microns.

4. The process of claim 3 wherein the discs are rotated at a relativevelocity between about 4000 and 8000 feet/minute.

5.,The process of claim 1 wherein the fibrous product is subjected to atotal power greater than about 0.2 kilowatt-hour/kilogram duringrefining.

6. The process of claim 1 wherein the polymer is an at least partiallycrystalline polyolefin.

7. The process of claim 1 wherein the polyolefin is polyethylene. It

' I 8. The process of claim 1 wherein the primary refining is carriedout under conditions to impart a vigorous fibrillating action to thefibrous product effective to produce a pulp having-a drainage factorgreater than v 110 second/ gram.

9. The process of claim 1 wherein the primary refining is carried outunder conditions to provide a pulp less than 90% by weight of which isretained on a 20 plus 35 mesh Tyler screens when passed therethrough ata consistency of 0.5% by weight.

1. IN A METHOD OF PRODUCING A PULP OF POLYOLEFIN FIBERS WHEREIN ANAQUEOUS DISPERSION OF A POLYOLEFIN IN A SOLVENT AT AN ELEVATEDTEMPERATURE AND PRESSURE IS FLASHED THROUGH A NOZZLE INTO A ZONE OFREDUCED PRESSURE TO FORM A FIBROUS PRODUCT, THE INPROVEMENT COMPRISINGDISSOLVING IN THE AQUEOUS PHASE OF THE DISPERSION FROM 0.75 TO 10% BYWEIGHT POLYVINYL ALCOHOL BASED ON THE WEIGHT OF THE POLYOLEFIN FLASHINGSAID DISPERSION TO FORM THE FIBROUS PRODUCT AND PASSING THE FIBROUSPRODUCT AT A TEMPERATURE BETWEEN ABOUT 10*C AND 60*C THROUGH A PRIMARYREFINING ZONE UNDER SUBSTANTIALLY ATMOSPHERIC PRESSURE IN THE PRESENCEOF THE POLYVINYL ALCOHOL UNDER CONDITIONS SUCH THAT A VIGOROUSFIBRILLATING ACTION IS IMPARTED TO THE FIBROUS PRODUCT.
 2. The processof claim 1 wherein the fibrous product is diluted with water to aconsistency between about 1% and 10% by weight prior to refining.
 3. Theprocess of claim 1 wherein refining is carried out by passing thefibrous material through a disc refiner with the spacing between thediscs set at between about 0 and 2500 microns.
 4. The process of claim 3wherein the discs are rotated at a relative velocity between about 4000and 8000 feet/minute.
 5. The process of claim 1 wherein the fibrousproduct is subjected to a total power greater than about 0.2kilowatt-hour/kilogram during refining.
 6. The process of claim 1wherein the polymer is an at least partially crystalline polyolefin. 7.The process of claim 1 wherein the polyolefin is polyethylene.
 8. Theprocess of claim 1 wherein the primary refining is carried out underconditions to impart a vigorous fibrillating action to the fibrousproduct effective to produce a pulp having a drainage factor greaterthan 1.0 second/gram.
 9. The process of claim 1 wherein the primaryrefining is carried out under conditions to provide a pulp less than 90%by weight of which is retained on a 20 plus 35 mesh Tyler screens whenpassed therethrough at a consistency of 0.5% by weight.